Typical melt spinning polymers, such as polyolefins, tend to be in a semi-crystalline state upon meltblown fiber extrusion (as measured by differential scanning calorimetry (DSC)). For polyolefins, this ordered state is due, in part, to a relatively high rate of crystallization and the extensional polymer chains oriented in the extrudate. In meltblown extrusion, extensional orientation is accomplished with high velocity, heated air in the elongational field. Extending polymer chains from the preferred random coiled configuration and crystal formation imparts internal stresses to the polymer. Provided the polymer is above its glass transition temperature (Tg) these stresses will dissipate. For meltblown polyolefins, the dissipation of stresses occurs spontaneously within a few days of web formation since the polymer's Tg is well below room temperature.
Melt blown polyethylene terephthalate (PET) generally exhibits a level of crystalline orientation commensurate with the strain level imparted during processing, and the time available for the polymer chains to relax during cooling. PET has a relatively slow rate of relaxation, a relatively low rate of crystallization, a relatively high melt temperature (Tm), and a glass transition temperature (Tg) above room temperature. The internal stresses from amorphous orientation within the elongational field are frozen-in place due to rapid cooling of the melt, thus retarding relaxation. As the Tg is approached and surpassed, the chains begin to relax. Annealing between Tg and Tm for a sufficient period of time allows the polymer to dissipate the internal stresses caused by elongational orientation, and for the chains to crystallize. The stress dissipation manifests itself in the form of shrinkage of the web's extruded dimensions, while crystallization of the polymer chains increases brittleness.
Efforts to provide a more stable and useful meltblown PET fiber, such as annealing a web while being held on a tentering structure has been described in U.S. Pat. No. 5,958,322 (Thompson et al.), or forming strain induced crystals during fiber attenuation as described in U.S. Pat. No. 6,667,254 (Thompson, Jr. et al.) and Japanese Kokai No. 3-45768. Other techniques for extracting soluble fractions of drawn crystalline polymer in a heated solvent and applying a tensile stress to provide for a stable polyester nonwoven fibrous web have been described in U.S. Pat. No. 3,823,210 (Hikaru Shii et al.), and by treating webs in solvent as described in U.S. Pat. No. 5,010,165 (Pruett et al.).